This invention relates to iris aperture control apparatus and, more particularly, to apparatus which is used either to control the iris aperture automatically as a function of the brightness level of a signal produced by the camera in which the iris is provided, or to maintain the iris aperture at a fixed size irrespective of changes in the brightness level of that signal, without requiring mechanical locking elements.
In photography, image sensing, image recording, and the like, the usual camera or image pick-up device generally is provided with an adjustable iris aperture. The size of the iris aperture determines the amount of light which enters the camera or pick-up device. As the amount of light increases, such as if the brightness of an image increases, the iris aperture should be reduced in size to avoid photographing or sensing an overly bright image. Conversely, if the brightness of the image decreases, the size of the iris aperture should increase.
Typically, in a video camera or image pick-up device, automatic control apparatus is provided for controlling the iris aperture. The brightness of the video signal produced by the camera is detected, and an iris drive mechanism is selectively energized as a function of that detected brightness level. For example, a selectively energizable motor, such as a DC motor, may be used to control the iris aperture, and this motor is controlled as a function of the detected brightness level of the video signal.
In some situations, the camera operator may wish to override the automatic iris control and maintain the iris aperture at a fixed size irrespective of changes in the brightness level of the signal produced by the camera. For example, if the same subject or scene is photographed or imaged for a long period of time, it may be desirable to lock the iris aperture so as to be insensitive to changes in the brightness of the subject. To provide this locking feature, some camera or image pick-up devices are provided with mechanical locking assemblies.
An automatic iris control device may be provided with a selectively energizable motor which is operated to increase or decrease the size of the iris aperture. In one type of locking assembly used therewith, a rubber element, such as a rubber disc, brake, or the like, is inserted into the transmission between the drive motor and the iris so as to prevent the iris from being driven. After several locking operations, however, the rubber device may be deformed and, as a consequence thereof, the reliability of this type of locking assembly may deteriorate over time. Moreover, if the automatic iris control device is of the type which automatically closes the iris when power to the camera or image pick-up device is interrupted, thereby protecting the pick-up screen thereof, such closure of the iris may be inhibited if the rubber element has been inserted to lock the iris at a fixed aperture. To protect the screen, it is necessary to unlock the iris when the camera power is interrupted. This additional control over the locking assembly adds to the complexity of the iris control device.
In another type of automatic iris control device, the aforementioned drive motor is mechanically coupled to the iris by a gear transmission. The iris may be locked to a fixed aperture size merely by meshing the gears to prevent rotation thereof. However, when the gears are brought into meshing engagement, it is highly possible that one of the gears must rotate by a maximum of one-half a tooth due to, for example, backlash. This results in a corresponding change in the iris aperture size. As a result, the actual aperture size at which the iris is locked may differ from the desired size by an amount corresponding to this one-half tooth. Although this deviation in the actual iris aperture size from the desired size thereof may not be readily noticeable when the iris is to be locked at a relatively large or wide aperture, this deviation becomes pronounced when the iris is to be locked at a relatively small aperture, such as when the iris almost is closed. Such a deviation at small iris aperture openings may greatly affect the level of the video signal produced by the video camera or image pick-up device.